Green Satin, also known as ARI 5851, was a
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
system developed by the
Royal Air Force
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...
as an
air navigation
The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another.
Successful air navigation involves piloting an air ...
aid. The system provided direct measures of the drift speed and direction, and thereby allowed accurate calculation of the winds aloft. These values were then fed into the
Navigation and Bombing System
The Navigation and Bombing System, or NBS, was a navigation system used in the Royal Air Force's V-bomber fleet. Primary among its parts was the Navigation and Bombing Computer (NBC), a complex electro mechanical computer that combined the fun ...
.
It was originally specified in 1949 as OR.3015 for the
English Electric Canberra
The English Electric Canberra is a British first-generation, jet-powered medium bomber. It was developed by English Electric during the mid- to late 1940s in response to a 1944 Air Ministry requirement for a successor to the wartime de Havil ...
bomber. The first examples arrived in 1953, and it was soon fit operationally. It was subsequently used on the
V bomber
The "V bombers" were the Royal Air Force (RAF) aircraft during the 1950s and 1960s that comprised the United Kingdom's strategic nuclear strike force known officially as the V force or Bomber Command Main Force. The three models of strategic ...
fleet. It remained in use on the V-bombers until the
Avro Vulcan
The Avro Vulcan (later Hawker Siddeley Vulcan from July 1963) is a jet-powered, tailless, delta-wing, high-altitude, strategic bomber, which was operated by the Royal Air Force (RAF) from 1956 until 1984. Aircraft manufacturer A.V. Roe and ...
s 1969 when it was replaced by the Decca 73.
Handley Page Victor
The Handley Page Victor is a British jet-powered strategic bomber developed and produced by Handley Page during the Cold War. It was the third and final '' V bomber'' to be operated by the Royal Air Force (RAF), the other two being the Avro ...
s may have used it until they left service in 1993.
The name comes from an era when the
Ministry of Supply
The Ministry of Supply (MoS) was a department of the UK government formed in 1939 to co-ordinate the supply of equipment to all three British armed forces, headed by the Minister of Supply. A separate ministry, however, was responsible for aircr ...
used
random combinations of colours and code words to prevent their
code name
A code name, call sign or cryptonym is a code word or name used, sometimes clandestinely, to refer to another name, word, project, or person. Code names are often used for military purposes, or in espionage. They may also be used in industrial c ...
s being too literal. A version known as Blue Silk with lower top-speed limits was used in some marks of Canberra and
Royal Navy
The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against F ...
aircraft.
Background
In
air navigation
The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another.
Successful air navigation involves piloting an air ...
there are six values of interest; airspeed, heading (the compass angle the nose is pointing), ground speed, course (the compass angle of the actual motion), wind speed and wind direction. Using any four of these values and basic
vector addition
In mathematics, physics, and engineering, a Euclidean vector or simply a vector (sometimes called a geometric vector or spatial vector) is a geometric object that has magnitude (or length) and direction. Vectors can be added to other vectors ac ...
, the other values can be determined through the
wind triangle
In air navigation, the wind triangle is a graphical representation of the relationship between aircraft motion and wind. It is used extensively in dead reckoning navigation.
The wind triangle is a vector diagram, with three vectors.
*The air ve ...
. Once determined, the path of the aircraft can be accurately calculated using
dead reckoning
In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
in comparison to an original fixed point.
Airspeed and heading can be measured with a fair degree of accuracy using onboard measurements, namely the
airspeed indicator
The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km/h, ...
and
gyrocompass
A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth (or another planetary body if used elsewhere in the universe) to find geographical direction automatically. The use of a gyroc ...
. Since the aircraft flies within the air mass, it is not possible to directly measure wind values, so their determination must be carried out with reference to some external measure. For most of the history of air navigation, this was accomplished through a reverse
dead reckoning
In navigation, dead reckoning is the process of calculating current position of some moving object by using a previously determined position, or fix, and then incorporating estimates of speed, heading direction, and course over elapsed time. ...
process, timing the passage of objects on the ground to measure ground speed, and either estimating the drift or measuring it with simple optical instruments like the
drift meter
A drift meter, also drift indicator and drift sight, is an optical device used to improve dead reckoning for aircraft navigation by measuring wind effect on flight.
Design
A drift meter consists of a small telescope extended vertically through th ...
. Both are inherently inaccurate, with the
US Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage of ...
suggesting such measures are accurate to only 10%.
Green Satin provided accurate and immediate measurements of the true ground speed and the drift angle. Combined with the airspeed and heading, air navigation complexity was greatly reduced. However, accurate dead reckoning also requires an accurate original "fix" from which future movements are determined. In practice,
Gee would be used to take a fix after the aircraft reached cruise altitude and speed, and would be used to fine-tune the navigation until it passed out of Gee range, perhaps 300 to 450 nautical miles.
Development
Green Satin started life in Operational Requirement (OR) 3015 in 1949. This called for a device to provide an accurate measure of the groundspeed of an aircraft to within 0.2 percent while flying at any speed between 100 and 700 knots at all altitudes up to 60,000 feet. It also had to measure the drift angle up to 20 degrees on either side of the heading, with an accuracy of less than 0.1 degree. It had to make these measurements over land or water, in any weather.
The solution to measuring ground speed was already well understood at the time, using a
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
system to compare the returned
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of two or more signals. Green Satin used four such signals to determine both ground speed and drift angle, which were sent out simultaneously by a single cross-shaped antenna with four feeds. In its neutral position, facing forward, one set of antennas sent out two signals aligned slightly fore and aft, and the other slightly to the port and starboard side. Each pair of signals was sent into a separate frequency
comparator
In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals V_+ and V_- and one binary digital output V_\text. The output is ideally
: ...
, with the output being the difference in frequencies represented by a
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
.
Consider an aircraft flying over the ground with no wind. In this case, the velocity to the side is zero, so the port and starboard signals will return to the aircraft at the same frequency. When sent through their comparator, the output is zero voltage, indicating no sideways drift. Now consider the case where the aircraft is being blown to the right. In this case, the starboard signal will be shifted upwards in frequency and the port signal downwards. The speed of the movement is not of interest, only the angle. To measure the angle, the entire antenna array was mounted on a motorized pivot and rotated back and forth until the output was zero again.
Once the drift angle is accurately measured, the antenna will be pointing directly in the direction of the drift. Any difference in frequency between the fore and aft pointing signals at this point is thus a measure of the ground speed.
Green Satin was a pulsed system, although it was not a
pulse-Doppler radar
A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars an ...
in the typical meaning. Pulses were used in order to allow a single antenna to be used for both broadcast and reception, not to measure distance (or in this case, altitude) through pulse timing. Frequencies were compared within a single pulse (a
monopulse radar
Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.
Monopulse radar avoids prob ...
), so the high frequency stability over longer multi-pulse times was not required. Green Satin was thus based on a simple
cavity magnetron
The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field while ...
generator, rather than the more frequency-stable solutions like the
klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
.
The electronics were housed within two pressurized canisters mounted on large backplanes that included power supplies and various electrical connections. These were mounted above the cabin area on the rear bulkhead of the Canberra. Some of these maintained their pressurization decades after being decommissioned.
Data from the Green Satin was fed into the
Navigation and Bombing System
The Navigation and Bombing System, or NBS, was a navigation system used in the Royal Air Force's V-bomber fleet. Primary among its parts was the Navigation and Bombing Computer (NBC), a complex electro mechanical computer that combined the fun ...
(NBS), a
mechanical computer
A mechanical computer is a computer built from mechanical components such as levers and gears rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment outp ...
that calculated the current location of the aircraft through the continual dead reckoning of the inputs. Most of these inputs were automatically fed from various aircraft instruments, but Green Satin data was normally fed in manually. The output of the NBS was the calculated latitude and longitude based solely on airspeed and heading, the values from the Green Satin were then added to these to produce the final outputs in the navigation computer. These outputs also drove correction circuits in the
H2S Mk. IX bombing radars and
Mark XIV bomb sight
The Mark XIV Bomb Sight was a bombsight developed by Royal Air Force (RAF) Bomber Command during the Second World War. It was also known as the Blackett sight after its primary inventor, P. M. S. Blackett. Production of a slightly modified ve ...
to adjust the predicted bomb trajectories.
Testing
In order to determine the operational accuracy of Green Satin, a lengthy series of test flights over land and water were carried out from an early Canberra. However, for these tests the G4B gyrocompass proved far too inaccurate as it required constant correction for drift. And while Gee was accurate enough for the purpose, taking a fix required lengthy manual calculation by the operator. The test program demanded additional equipment to solve both of these problems.
Accurate heading information was supplied by the Azimuth Datum Instrument (ADI), a
star tracker
A star tracker is an optical device that measures the positions of stars using photocells or a camera.
As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may ...
mounted on a
periscope
A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position.
In its simplest form, it consists of an outer case with ...
so that it projected its display on a plate in front of the navigator. Using angle measurements similar to those taken with a
sextant
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celes ...
, the aircraft heading could be accurately determined. For initial position fixes, Gee was replaced with the
Decca Navigator System
The Decca Navigator System was a hyperbolic navigation, hyperbolic radio navigation system which allowed ships and aircraft to determine their position by using radio signals from a dedicated system of static radio transmitters. The system used ...
(Mark 6), which directly output measurements on three dials.
By locating both the ADI and Decca output on the navigation console, along with the Green Satin outputs, a
movie camera
A movie camera (also known as a film camera and cine-camera) is a type of photographic camera that rapidly takes a sequence of photographs, either on an image sensor or onto film stock, in order to produce a moving image to project onto a movie s ...
was able to record everything in real time for later data analysis on the ground. This consisted of taking a snapshot of the settings every six seconds and then averaging the instrument positions. Testing demonstrated that accuracy of Green Satin over land in straight and level flight was less than ±0.1% of distance flown, and less than ±0.1 of a degree in drift.
In use
The Green Satin was initially used with a simple display system with two large dials presenting true ground speed in knots on the left, and drift angle on the right. The output from this system was typically sent, along with the G4 compass' output, to the Mark 4 Ground Position Indicator (GPI). The GPI was a simple
mechanical computer
A mechanical computer is a computer built from mechanical components such as levers and gears rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment outp ...
that integrated the inputs to produce an offset from a user-provided initial location (taken from Gee, for instance), and presented this as either latitude and longitude or grid reference numbers on two
odometer
An odometer or odograph is an instrument used for measuring the distance traveled by a vehicle, such as a bicycle or car. The device may be electronic, mechanical, or a combination of the two (electromechanical). The noun derives from ancient Gr ...
-like displays.
Green Satin initially equipped the Canberra force, but was soon used on most larger RAF aircraft. Problems locking on to a calm sea surface meant Decca navigational equipment had to be used instead during
Grapple bomb aiming from
Valiant aircraft.
Other applications
Green Satin was also used in combination with a second radar system named Red Setter to produce a
side looking airborne radar
Side-looking airborne radar (SLAR) is an aircraft- or satellite-mounted imaging radar pointing perpendicular to the direction of flight (hence ''side-looking''). A squinted (nonperpendicular) mode is possible also. SLAR can be fitted with a sta ...
(SLAR) system. This used the antenna and electronics from an
H2S Mark IXA, but turned off the scanning motor so it normally pointed to one side of the aircraft. The drift angle from the Green Satin was fed into the motor to make minor adjustments to keep the antenna aligned at right angles to the aircraft's ground track, independent of the heading.
The CRT display used by the normal H2S was fitted with a film system with a motorized takeup reel. The motor was connected to the ground speed output of the Green Satin, so it drew the film across the front of the display at a set speed independent of the airspeed. With the scanning motor turned off, the display on the CRT was a single line, modulated in brightness by the returns at various distances from the aircraft. This exposed the film, producing a 2D image of the land to one side of the aircraft.
The film was pulled through a rapid developer system produced by
Kelvin Hughes
Hensoldt UK, formerly Kelvin Hughes, is a British company specialising in the design and manufacture of navigation and surveillance systems and a supplier of navigational data to both the commercial marine and government marketplace.
The company ...
, who made similar systems for a variety of purposes. The film was processed and ready for viewing after a short delay.
See also
*
AN/APN-89, the US equivalent of Green Satin.
Notes
References
Citations
Bibliography
*
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{{refend
Aircraft radars
Military radars of the United Kingdom
Navigational aids